Electromagnetic forming apparatus

ABSTRACT

An electromagnetic forming apparatus utilizes a forming coil having separable die portions for receiving a workpiece which is configured in such a manner that it could not otherwise be worked by a high intensity magnetic field. An improved design of the forming coil increases its operating life.

United States Patent Brower [151 3,654,787 {451 Apr. 11, 1972 [54]ELECTROMAGNETIC FORMING APPARATUS [72] Inventor: David F. Brower, SanDiego, Calif.

[73] Assignee: Gulf Oil Corporation, San Diego, Calif.

[22] Filed: Oct. 15, 1968 [21] Appl. No.: 767,777

[52] US. Cl..... ..72/56 [51] Int. Cl ..B2ld 26/14 [58] Field of Search..72/56; 330/225, 229, 232

[56] References Cited UNITED STATES PATENTS 3,347,074 10/1967 Eilers etal. ..72/56 Primary Examiner-Richard J. l-lerbst Attorney-Fitch, Even,Tabin & Luedeka [5 7] ABSTRACT An electromagnetic forming apparatusutilizes a forming coil having separable die portions for receiving aworkpiece which is configured in such a manner that it could nototherwise be worked by a high intensity magnetic field. An improveddesign of the forming coil increases its operating life.

4 Claims, 5 Drawing Figures TO ENERGY SOURCE PAIENTEBAPRH 1912 3,654,787

SHEET 1 OF 2 Mil 1 w r llllllI INVENTOR DAVID F. BROWER PATENTEDAPR 11I972 3, 654,787

sum 2 or 2 F I36 I50 TO ENERGY SOURCE INVENTOR DAVID F. BROWERELECTROMAGNETIC FORMING APPARATUS The present invention relatesgenerally to forming apparatus, and more particularly to apparatus bymeans of which conductive materials may be formed or swaged by energyacquired from a varying magnetic field.

Various methods and apparatus have heretofore been developed for formingor swaging conductive materials by employing a varying magnetic field ofhigh intensity. In such methods and apparatus generally, an electricalcurrent pulse of high amperage and short duration is passed through aconductor, typically formed into a work coil, to thereby produce apulsed magnetic field of high intensity in the proximity of theconductor. A conductive workpiece is positioned in the pulsed magneticfield and a current pulse is thereby induced in the workpiece, whichcurrent pulse interacts with the magnetic field to produce a forceacting on the workpiece. This force or magnetic pressure is madesufficiently great to cause the desired deformation of the workpiece,the swaging of one piece to another, etc. The manner of deformation isgenerally dependent upon the shape or configuration of the magneticfield and the position of the workpiece relative to the field. Repeatedpulses of current may be applied to the conductor or coil, thus causinga series of deformingimpulses to be applied to the workpiece.

As indicated above, such apparatus and methods for electromagneticforming of a workpiece typically employ a conductor in the form of acoil, i.e., a work or forming coil, which surrounds that part of theconductive or metallic workpiece to be formed or swaged. For example, inthe typical case of magnetic swaging, it is generally possible to insertan elongated uniform workpiece into the forming coil aperture or openingwhich defines the magnetic field and controls the pressure pattern onthe workpiece. However, when using such a forming coil with certaintypes or shapes of workpieces, a problem may arise in that the workpiececannot be inserted within the coil aperture or the formed assemblycannot be withdrawn therefrom. This problem may arise in connection withworkpieces having a generally dumbbell shape, wherein the end portionsmay be flanged or otherwise dimensioned so as to be larger than the coilaperture, making it impossible to insert or pass the workpiecetherethrough, while the aperture is sufficiently small to provide anefficient forming operation. Further, other applications and workpiececonfigurations prevent the workpiece from being inserted axially intothe forming coil, and examples of such configurations are those whichare closed in themselves, such as a ring, or which, for some otherreason, cannot be threaded into the coil. Examples of this latter typeare swaging applications requiring work on pipelines or electricalconductors which, by reason of their indefinite or long lengths, ortheir installations, cannot be threaded through a coil.

Accordingly, it is an object of the present invention to provideapparatus for electromagnetically swaging or otherwise forming suchworkpieces which are not practicably formable by means of the type offorming coils generally described above.

A further object is to provide an improved magnetic form ing apparatuswhich is efficient in operation, relatively inexpensive to manufacture,durable in use, and yet may be used to form such cumbersome workpiecesas described.

Other objects and advantages of the present invention will becomeapparent by reference to the following description and the accompanyingdrawings, in which:

FIG. 1 is a view partially in elevation and partially schematic of aforming apparatus in accordance with one embodiment of the presentinvention, showing the apparatus in its operating condition;

FIG. 2 is a partial elevational view of the apparatus of FIG. 1, showingthe apparatus in its loading and unloading condition;

FIG. 3 is a sectional view taken along lines 3-3 in FIG. 2;

FIG. 4 is an elevational view, partially broken away, of a portion of aforming apparatus in accordance with a further embodiment of theinvention, showing the apparatus in its operating condition; and

FIG. 5 is a partial elevational view of the apparatus of F I67 4,showing the apparatus in its loading and unloading condition.

Referring now to FIGS. 1 through 3, apparatus is shown forelectromagnetically forming a metallic workpiece 10, illustrated as apipe or tube 12 having a collar 14 generally concentrically arranged anddisposed thereover so that it can be swaged to the tube. The tube 12may, for example, be fixed in place with no access to its ends, or mayhave flanges or fittings on its ends which extend radially beyond theperiphery of the collar 14. The fonning apparatus comprises a forming orwork coil assembly 15 having a pair of relatively movable and engageabledies or coil-halves l6 and 18 formed of a conductive material, which areso arranged, when engaged as shown in FIG. 1, that they provide currentpaths around the workpiece 10. Operating and clamping means 20 isprovided for maneuvering the dies 16 and 18 into and out of engagement,and an energy source 22, connected directly to the dies 16 and 18, isoperative to supply high amperage current pulses to the dies in theirengaged position so that the current flow through the dies creates highintensity magnetic fields for forming the workpiece 10 by, for example,swaging the collar 14 onto the tube 12.

The pair of relatively movable and engageable dies 16 and 18 haveinterfacing surfaces 26 and 28, respectively, which cooperate to definea common recess or workspace 24 to receive the workpiece l0, and thedies are arranged to form a single turn conductor for electric currentaround the recess 24 when the dies are engaged. The interfacingengageable and disengageable surfaces 26 and 28 are symmetrically formedto define the common recess 24 for the workpiece and may be suitableinsulated from each other, as will be hereinafter described.

The energy source 22 shown schematically in FIG. 1, is connecteddirectly to dies 16 and 18 by means of a transmission line assembly 29which supplies the high amperage current pulses to the dies withsuitable polarity or direction of current flow to produce additive highintensity magnetic fields within the recess or workspace 24 for formingthe workpiece. The current pulses to each die follow a path around therecess 24 from one conductor to the other conductor of the transmissionline assembly 29, through suitable interface contacts which will belater described.

The workpiece 10, comprising the conduit or tube 12 and the collar 14,is preferably suspended within the workspace 24 so as to provide agapbetween the outer or peripheral surface of the collar and the innersurface or wall of the coil aperture. The collar 14 may be disposed onthe conduit with some small clearance therebetween, although theparticular fit used will generally depend on the material type andthickness, and on the operating parameters employed. The gap between thecollar and the coil may be filled in some cases with a solid insulationor insulating film.

As shown in FIG. 2, the dies 16 and 18 are illustrated in their loadingand unloading condition wherein they are disengaged or open forintroduction or positioning of the workpiece 10 into the workspace 24 orfor the removal of the completed or swaged assembly. The die 18, whichis fixed, and the transmission line assembly 29 form an integral unit,as shown. The transmission line assembly 29 has a pair of conductorleads or plates 30 and 32 which are separated by an insulating layer orsheet 34, and serve as input and output current terminals for the workcoil 15. The input or upper lead or bus 30, which is desirably in theform of a flat plate, strip, or bar has a generally cylindrical orsemi-cylindrical contact surface 36 disposed near the end thereofadjacent the die 18 and fixed thereto. The contact surface 36, asillustrated, is formed by a generally circular cylindrical conductivemetal bar or rod which is welded or brazed into a complementary hollowor groove 38 in the bus 30. The output or lower lead or bus 32, whichmay be in the form of a flat plate, strip or bar, is joined to the die18 at surface 40 with a high conductivity joint, made in any knownmanner. The insulation 34, which may be of any suitable type, isdisposed between the buses 30 and 32, and may extend to the workspace24, although it need only extend to the contact end of the upper bus 30to provide support therefor. The insulation between this end of the bus30 and the workspace 24 may be formed by an air gap between the diesurfaces 26 and 28 in this region.

The portion of the surface 28 on the opposite side of the workspace 24has a second generally cylindrical or semi-cylindrical contact surface42 formed in a manner similar to the first contact surface 36, and has agenerally circular cylindrical rod welded or brazed in a mating hollow44 in the die 18.

The corresponding surface 26 of the movable die 16 has generallycomplementary contact portions 46 and 48 which define contact surfaceswith the fixed contacts 36 and 42, respectively. Thus, a completecurrent path is provided from the input terminal bus 30, through contactsurfaces 36 and 46, die 16, contact surfaces 48 and 42, and die 18, tothe output terminal bus 32.

The output of the energy source 22 is connected directly to the buses 30and 32 of the transmission line assembly 29. The energy source 22 mayinclude any suitable form of circuitry commonly utilized withelectromagnetic forming apparatus, and, in the illustrated embodiment,the source includes a dc. power supply 50 which is serially connectedwith a switch 52, a current limiting resistor 54, and a storagecapacitor 56 to form a charging circuit for the capacitor. Thetransmission line assembly 29, and consequently the coil 15, isconnected across the capacitor 56 through an impedance matchingtransformer 58 and a firing switch 60. Thus, in operation, the chargingswitch 52 is closed until a sufficient charge is attained on thecapacitor 56, after which the charging switch 52 is opened and thefiring switch 60 is closed. This discharges the stored charge to thework coil 15. More particularly, the switch 60 may comprise a suitablethyratron or ignitron circuit which is selectively fired to dischargethe capacitor 56 through the transformer 58 which supplies a highmagnitude, short duration current pulse through the dies 16 and 18, andaround the workspace 24, by applying a positive potential to inputterminal bus 30 and a negative potential to the output terminal bus 32.An insulating material or air gap is provided, as previously mentioned,to prevent the work coil from constituting a closed loop or shortedturn. The contact surface portions 36, 46 and 42, 48 of the surfaces 26and 28 on opposite sides of the recess 24 are in electrical contactingengagement, but the remaining portions thereof are electricallyinsulated from each other by an air gap 62 (FIG. 1) or by an insulatingsheet, such as the extended portion 64 of insulation 34.

The large current pulse flowing in the single turn formed by the dies 16and 18 generates an intense varying magnetic field in the workspace 24which induces a current in the collar 14. This induced current interactswith the generated magnetic field to produce a very high opposing radialforce or pressure between the collar and the coil assembly, causing thecollar 14 to be swaged onto the tube 12. A suitable mandrel or rod may,if desired, be inserted into the tube 12 for additional support duringthe swaging operation and to maintain the tube dimensrons.

The discharge current from the capacitor 56 may, of course, bedischarged through the coil 15 via transmission line assembly 29 withoutthe matching transformer 58. However, the use of the transformer 58 tomatch impedances, may increase the efficiency of the energy transfer ifthe impedance of the work coil is other than optimum for the givencapacitor bank.

The dies 16 and 18 may each comprise a field shaper portion 66 and 68,respectively, concentrically disposed therewithin, which together definethe workspace orifice 24. The field shaper portions 66 and 68 arecomposed of conductive metallic material of high strength, and may bemounted within the outer die portions by any suitable means so that theyare either in direct conductive relation thereto or inductively coupledthereto. That is, they may be insulated from the outer conductive dieportions so as to receive current by induction or transformer actionrather than by conduction. Alternatively, of course, the dies 16 and 18may be each formed as a unitary mass extending to and defining theworkspace 24.

Although the thickness of the dies 16 and 18 (i.e., in the directionparallel to the axis of the aperture or orifice 24) may be uniform, itis sometimes desirable for certain applications to reduce the effectivethickness in the region about the orifice 24, as compared to thethickness of the outer portions. As shown in FIG. 3, the dies have atapered portion between the peripheral region and the workspace so thateach die is quite narrow in the workspace to produce the necessary highpressures required for forming. The cross-section then diverges towardsthe general region of the contacts, with the contacts being preferablyabout four times as wide as the working region. This serves to increasethe magnetic force on the workpiece in the workspace for a given currentdensity at the contact area.

The forming coil 15 is supported on a sturdy cabinet or frame structure70 which resists the reaction forces produced when the coil is pulsed,and supports the workpiece to the extent that it is not otherwisesupported. The frame 70 has a lower arm 72 which extends from the lowerportion thereof and receives the lower or stationary die 18 which issuitably insulated from the arm 72 by insulation 74. An upper arm 76extends from the upper part of the frame 70, and a C shaped bracket 78is rigidly fixed to the cantilevered end thereof. The bracket 78 has alower sleeve portion 80 and an upper ear portion 82 formed integrallytherewith. A rigid shaft or rod 84 slidably engages the sleeve portion80 for vertical motion therewithin. The lower end of the shaft 84 iscoupled to the movable die 16 by any suitable means such as threadedassembly 86, which also desirably may insulate the movable die 16 fromthe shaft 84 by the use of an intermediate section of insulatingmaterial, or in any other convenient manner. A handle 88 is pivotablyfastened to the ear portion 82 of the bracket 78 at pivot 90, which ispreferably located in alignment with the slidable shaft 84. A link 92has one end pivotably connected to a point 94 on the handle which isspaced radially from the handle pivot 90, and the other end of the link92 is pivotably fastened at 96 to the upper end of the slidable shaft84. Consequently, when the handle 88 is lifted or rotated in acounterclockwise direction, the linkage pulls the movable die 16 to itsopen or disengaged position for loading or unloading a work piece. Whenthe handle 88 is lowered in the clockwise direction, the linkage lowersthe movable die 16 into engagement with the stationary die 18. When themovable die 16 has reached its lowest or engaged position, continuedmovement clockwise of the handle 88 clamps the dies together, since theupper link pivot 94 moves into alignment with, or to the left of, theslidable shaft 84, and the handle 88 cannot move from this position bythe application of the upward vector force produced by the reaction tothe magnetic pressure at the workpiece. Thus, the dies 16 and 18 canonly be unclamped by lifting the handle so that the upper link pivot 94swings to the right of the slidable shaft axis. As previously mentioned,the clamping action on the dies 16 and 18 is necessitated by theproduction of opposing reaction forces on pulsation of the work coil 15,which tends to drive the dies apart. The handle and linkage assemblydescribed, provides a simple means maneuvering the dies into and out ofengagement, although other mechanisms may be employed for this purpose.

Referring now to FIGS. 4 and 5, there is shown a further embodiment ofthe electromagnetic forming apparatus of the invention, and partscorresponding to those in FIGS. l-3 are indicated with like referencecharacters, but having the suffix a."The forming coil 15a is shown inFIG. 4 in its closed or engaged position, and may be supported from theapparatus frame (not shown) by conductive member 100. The pair ofrelatively movable dies 16a and have complementary engaging faces 26aand28a (FIG. 5) which define the recess or workspace 24a. A transmissionline assembly 29a is connected to a pulse energy source (not shown) ofthe same type as illustrated in FIG. 1. The transmission line assembly29a comprises an input terminal conductor or bus 30a, an output terminalconductor or bus 32a, and an insulating sheet 34a disposed therebetween.Rigid conductive members 100, 102 and 104 are provided for structuralstrength, the former being at one potential and the latter two being ata different potential when the coil a is pulsed to produce a pulsecurrent through the coil dies. The conductor 30a is of a flexible typeand the conductive members 102 and 104 are spaced apart in the region ofconductor portion 106 to permit the movable die 16a to be pivoted fordisengagement of the surfaces 26a and 28a so that a workpiece may beintroduced into or removed from the workspace 24a. In this manner, thedies are disengaged by a tilting motion of the supper die 16a, andcomplete separation of the dies is avoided.

The complementary surfaces 26a and 28a define electrical contactsurfaces 108 and 110, respectively, which are in contact when the dies16a and 18a are in engagement. The contact surface 108 of die 16a isformed by a fingerlike projection defined on one side by the workspacerecess and on the other side by a hollow 112. This finger-likeprojection is dimensioned so as to be slightly flexible for a purpose tobe explained later. The other contact surface 110 of die 18a is formedby a projection 114 from the complementary surface 280. Projection 114is dimensioned so as to be rigid relative to the projection mentionedabove. The projection 114 has an inclined contact face and nests in thehollow 112 with the surface 108 pressed against the inclined contactface when the dies are in engagement, as shown in FIG. 4. An alignmentpin 116 protrudes from the surface 28a of the stationary die 18a andfits into a mating recess or bore 118 in the surface 26a of the movabledie 16a. A clamping means 120 is provided which comprises an angularshaped arm 122 which has a relatively short portion 124 and a relativelylonger portion 126. The end of the shorter arm portion 124 is pivotablyfastened to the lower or stationary die 18a at a pivot point 127directly beneath the workspace 24a. The longer arm portion 126 has ablock 128 pivotably fastened to the upper end thereof at pivot 130. Theblock 128 has a threaded bore, and a threaded screw 132 is disposed inthe bore and extends through the pivotable block 128 so that the screwmay be turned downwardly until the tip 134 enters or engages a recess136 in the outer top surface of the movable die 16a. The recess 136 isdiametrically in line with the workspace 24a and the lower pivot point127, and the screw 132 is aligned, due to the relative lengths of thearms and their angular relationship, to preferably be directly over theworkspace 24a and in line with the lower pivotable connection.

In order to permit the pair of dies to be opened or disengaged from eachother for the insertion or removal of a workpiece, the screw is threadedupwardly until the lower tip thereof is disengaged from the recess, andthen the entire clamping assembly 120 is pivoted counterclockwise or tothe left (as illustrated in FIGS. 4 and 5). This unclamps the upper die16a from the lower die 18a and permits the upper die to be raised in themanner illustrated in FIG. 5. Since the upper conductor or supply lead30a is flexible, it permits the pivoting action by bending. The movabledie 16a may be maneuvered out of and into engagement with the stationarydie 18a by merely lifting the die 16a by hand or by any suitablemechanism, either manual or motor driven.

Thus, the coil 150 may be separated to permit the ready removal orinsertion of a general workpiece, but yet only one electrical contact isrequired to complete the current path between the separable dies, ratherthan the two contacts required in the embodiment of FIGS. 1 through 3.This is an additional advantage in that it is believed that contactfailure is a principal cause of coil failure, and the elimination of oneof the two contacts will generally increase the lift of the coil, i.e.,increase the number of coil pulses before coil failure. The reactionproduced to each pulse tends to separate the coil dies, which may causearcing and burning of the interface contacts. The contact surfaceconfiguration 108 and results in minimal surface separation in responseto pulsing, thus resulting in less arcing and longer contact life. Theexpanding magnetic forces within the workspace 24a act to exert aradially outward bending moment to the flexible finger-like projectioncontaining the bearing surface 108 so as to greatly increase the contactpressure during the magnetic pulse, resulting in lower contactresistance and thus further lessening of the burning and pitting so asto increase the contact life. These contact portions may, of course, bereversed, with the flexible contact associated with or formed in thestationary die and the rigid contact portion associated with or formedin the movable die, rather than as shown. The coil life will thereforegenerally be increased somewhat from both the longer contact life andthe use of a contact on only one side rather than on both sides of theworkspace.

Thus, there has been described a method of electromagnetically forming aworkpiece comprising the steps of installing the workpiece within arecess provided within at least one of two relatively movable diesformed of a highly conductive material, moving the dies intoelectrically insulated engagement so as to provide individualproximately parallel current paths, each extending around a portion ofthe workpiece, and connecting the dies individually to a source of highamperage opposite polarity current pulses so that the current flowingalong the current path in one die is in the opposite direction of thecurrent flowing in the other die so that additive high intensitymagnetic fields are produced for forming the workpiece. The inductivedies closely surround the workpiece for effective magnetic couplingthereto and serve as conductors for the current flowing around theworkpiece, the dies defining a recess for the workpiece when they are inengagement with each other. In this position the dies form a single turnconductor for the current since they are electrically isolated in oneportion thereof to prevent their forming a continuous conductive loop.The coil dies may be satisfactorily made from copper, molybdenum orother highly conductive materials or combinations thereof. Also,suitable cooling tubes may be provided in the coil to remove the heatgenerated therein.

Although particular types of manual clamping and die maneuvering oroperating devices have been herein described for clamping and unclampingthe pair of conductive dies and for engaging and disengaging one fromthe other, it is understood that other forms of devices for thesepurposes may be used. Such devices may be motor driven and supplied by asource of energy and/or automatically operated, and thus, for example,may conveniently include an electrical solenoid, a hydraulic system,etc., as desired. Such a mechanism may, for example, have the connectingrod of a hydraulic piston assembly coupled to the handle 88 of theembodiment illustrated in FIGS. 1 through 3 through any suitable linkagearrangement. Further, the principles of the invention may be applied, ofcourse, to other forming or metal working techniques than thosespecifically described herein, such as to various shaping operations.

Various modifications of the present invention will be apparent to thoseskilled in the art; and accordingly, the invention should be defined bythe appended claims, and equivalents thereof.

Various features of the invention are set forth in the following claims.

What is claimed is:

l. A work coil for electromagnetically forming a metallic workpiece, andadapted for connection to a high amperage electric current supply,comprising a pair of relatively movable and engageable dies formed of aconductive material and having opposing interface surfaces withrespective recesses cooperating to define a workspace for the workpieceand arranged to form a single turn for electric current around theworkspace when the dies are engaged, a first electric contact portionassociated with the interface surface of one of said dies on one side ofsaid workspace formed by a projecting surface generally transverse tosaid interface surfaces, and a from said contact portions, and meansassociated with one of 10 said terminals to permit opening of said diesfor the introduction or removal of a workpiece.

2. The work coil of claim 1 wherein one of said contact portions isrigid and the other of said contact portions is relatively flexible toprovide an increased contact pressure between their respectivecontacting surfaces on pulsing of the coil.

3. The work coil of claim 2 wherein said rigid contact portion has aninclined projecting surface, said relatively flexible contact portionhas a finger-like projecting surface, and said flexible contact portionis radially closer to the workspace than said rigid contact portion.

4. Apparatus for electromagnetically forming a metallic workpiece,comprising a pair of relatively movable and engageable dies formed of aconductive material and having the interface surfaces thereofcooperating to define a common recess for the workpiece and arranged toform a single turn for electric current around the recess when the diesare engaged,

a first electrical contact formed by a slightly flexible fingerlikeprojection from the interface surface of one of said dies on one side ofsaid recess, and a second electrical contact formed by a relativelyrigid projection from the interface surface of the other of said diesalso on said one side of said recess and so arranged as to be inengagement with the fingerlike projection when the dies are inengagement with one of said projections being nested within a hollowlocated adjacent the other of said projections, a first terminalconnected to one die in the region of the interface on the opposite sideof the recess from said contacts, a second terminal connected to theother die on said opposite side of the recess, insulating means disposedbetween said first and second tenninals to prevent electrical contacttherebetween, means associated with one of said terminals to permitpivoting of the die to which it is connected so as to enable theworkpiece to be introduced in or removed from said recess, and an energysource connected to the dies through said first and second leads tosupply a high amperage electric current pulse thereto so that thecurrent around the single turn creates a high intensity magnetic fieldwithin said recess for forming the workpiece and said magnetic fieldwithin said recess acts to exert a radially outward bending moment onthe flexible finger-like projection to increase the contact pressureagainst the rigid projection during the magnetic pulse.

1. A work coil for electromagnetically forming a metallic workpiece, andadapted for connection to a high amperage electric current supply,comprising a pair of relatively movable and engageable dies formed of aconductive material and having opposing interface surfaces withrespective recesses cooperating to define a workspace for the workpieceand arranged to form a single turn for electric current around theworkspace when the dies are engaged, a first electric contact portionassociated with the interface surface of one of said dies on one side ofsaid workspace formed by a projecting surface generally transverse tosaid interface surfaces, and a second electric contact portionassociated with the interface surface of the other of said dies on saidone side of the workspace formed by a further projecting surfacegenerally transverse to said interface surfaces, a hollow locatedadjacent to one of said projecting surfaces and disposed so that theother of said projecting surfaces is nested within said hollow and incontact with said one projecting surface when said dies are in mutualengagement, first and second terminals respectively connected to saiddies on the opposite side of the workspace from said contact portions,and means associated with one of said terminals to permit opening ofsaid dies for the introduction or removal of a workpiece.
 2. The workcoil of claim 1 wherein one of said contact portions is rigid and theother of said contact portions is relatively flexible to provide anincreased contact pressure between their respective contacting surfaceson pulsing of the coil.
 3. The work coil of claim 2 wherein said rigidcontact portion has an inclined projecting surface, said relativelyflexible contact portion has a finger-like projecting surface, and saidflexible contact portion is radially closer to the workspace than saidrigid contact portion.
 4. Apparatus for electromagnetically forming ametallic workpiece, comprising a pair of relatively movable andengageable dies formed of a conductive material and having the interfacesurfaces thereof cooperating to define a common recess for the workpieceand arranged to form a single turn for electric current around therecess whEn the dies are engaged, a first electrical contact formed by aslightly flexible finger-like projection from the interface surface ofone of said dies on one side of said recess, and a second electricalcontact formed by a relatively rigid projection from the interfacesurface of the other of said dies also on said one side of said recessand so arranged as to be in engagement with the finger-like projectionwhen the dies are in engagement with one of said projections beingnested within a hollow located adjacent the other of said projections, afirst terminal connected to one die in the region of the interface onthe opposite side of the recess from said contacts, a second terminalconnected to the other die on said opposite side of the recess,insulating means disposed between said first and second terminals toprevent electrical contact therebetween, means associated with one ofsaid terminals to permit pivoting of the die to which it is connected soas to enable the workpiece to be introduced in or removed from saidrecess, and an energy source connected to the dies through said firstand second leads to supply a high amperage electric current pulsethereto so that the current around the single turn creates a highintensity magnetic field within said recess for forming the workpieceand said magnetic field within said recess acts to exert a radiallyoutward bending moment on the flexible finger-like projection toincrease the contact pressure against the rigid projection during themagnetic pulse.